Donald Trump claimed on Monday, without offering evidence, that pardons signed by Joe Biden were “void, vacant and of no further force and effect” because they were signed with an autopen.

The Merriam-Webster dictionary defines an autopen as “a device that mechanically reproduces a person’s signature”.

Trump made his claim in a post to his social media platform that used his abusive nickname for Biden and referred to members of the House January 6 committee, writing: “The ‘Pardons; that Sleepy Joe Biden gave to the Unselect Committee of Political Thugs, and many others, are hereby declared VOID, VACANT, AND OF NO FURTHER FORCE OR EFFECT, because of the fact that they were done by Autopen.

“In other words, Joe Biden did not sign them but, more importantly, he did not know anything about them!”

Close aides to Trump have said his own authorship of Truth Social posts can be detected by factors including the free use of capital letters.

Trump also made the claim aboard Air Force One on Sunday, telling reporters: “It’s not my decision – that’ll be up to a court – but I would say that they’re null and void, because I’m sure Biden didn’t have any idea that it was taking place, and somebody was using an autopen to sign off and to give pardons.”

Trump’s claim came after the Heritage Foundation, a pro-Trump rightwing thinktank, released a report into Biden’s purported autopen use.

Court precedents suggest Trump was wrong to say a president must personally sign any pardon, reprieve or other act of clemency.

In 1929, the US justice department held that “it is wholly for the president to decide” the method by which a pardon is handed down. Last year, a federal appeals court said presidential pardons do not even have to be in writing.

On 20 January, his last day in office, Biden gave pre-emptive pardons to family members and all members of the House committee that investigated the deadly January 6 attack on Congress, which Trump incited in his attempt to overturn his defeat by Biden in the 2020 election.

Two Republican Trump opponents, Liz Cheney of Wyoming and Adam Kinzinger of Illinois, sat on the committee. Both have since left Congress.

In his social media post, Trump wrote: “The necessary Pardoning Documents were not explained to, or approved by, Biden. He knew nothing about them, and the people that did may have committed a crime.”

Repeating unsubstantiated rightwing claims about the committee’s treatment of records, he continued: “Therefore, those on the Unselect Committee, who destroyed and deleted ALL evidence obtained during their two year Witch Hunt of me, and many other innocent people, should fully understand that they are subject to investigation at the highest level.”

Members of the January 6 committee said they did not want pardons from Biden, stressing they had done nothing wrong. Nonetheless, fear of revenge investigations and prosecutions from Trump remains high, particularly as his administration has targeted government lawyers who investigated him in his two federal criminal cases and outside lawyers with past links to Democrats.